The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the speed of changing the transmission ratio in accordance with driving conditions of the vehicle.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
When starting of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (to upshift). At that time if the engine speed is kept constant, the transmission ratio is automatically and continuously reduced at a speed which is determined by line pressure, the pressure of oil supplied to the servo device of the drive pulley, and the actual transmission ratio. In such a system, the rate of change of the transmission ratio (hereinafter called transmission ratio changing speed) up to a desired transmission ratio can not be controlled in accordance with driving conditions. Accordingly, hunting or overshooting of the of transmission ratio occurs, which decreases the driveability of the vehicle.
Japanese Patent Laid Open No. 59-217047 shows a system which operates to control the transmission ratio changing speed with a constant increment .DELTA.e in accordance with the deviation of the actual transmission ratio from a desired ratio. However, if the value of the increment .DELTA.e is increased in order to quickly control the transmission ratio, overshooting of the control operation occurs. To the contrary if the value .DELTA.e is reduced in order to prevent the overshooting, the response of the control system deteriorates.